Microstructure Evolution And Of Cu-20Ni-20Mn Alloy With High-strength And High-elasticity, And Its Effects On Alloy Properties

As a high-strength and high-elasticity copper alloy without beryllium, Cu-20Ni-20 Mn alloy had the similar elasticity modulus and strength compared with beryllium bronze, in addition, the high temperature stability of Cu-20Ni-20 Mn alloy was superior to beryllium bronze. It was a potentially substitute of beryllium bronze. So far, the aging process of Cu-20Ni-20 Mn alloy was illuminated preliminary by studying the phase diagram of the Cu-Ni-Mn system and microstructure evolution of Cu-20Ni-20 Mn alloy.In order to further study the microstructure evolution and strengthening mechanism of Cu-20Ni-20 Mn alloy during the aging treatment, the microstructure was observed and analyzed by optical microscopy and electron microscopy and X-Ray Diffraction, and the mechanical properties were measured by tensile test, impact test and wear test. The mapping relationship of microstructure, strengthening mechanism and mechanical property in Cu-20Ni-20 Mn alloy was built by analyzing the interrelationship between microstructure and mechanical property. The regulation mechanisms of microstructure characteristic to the comprehensive mechanical properties in Cu-20Ni-20 Mn alloy under different service conditions were expound.Discontinuous precipitation reaction, continuous precipitation reaction and dissolution reaction of precipitation were found in Cu-20Ni-20 Mn alloy. The temperature ranges of three solid phase transformations were established by OM, SEM and DSC. In the range 200℃ to 475℃ discontinuous precipitation was observed. In the range 350℃ to 475℃ continuous precipitation was found. The solvus temperature was between 500℃ and 525℃.The discontinuous precipitation colony with a lamellar mixed structure consisting of alternating lamellae of solute-depleted matrix and Ni Mn phase formed at the original grain boundaries and advanced into the grain interiors during discontinuous precipitation. Because of the high strength of discontinuous precipitation colony, the strength of Cu-20Ni-20 Mn alloy increased.The discontinuous precipitation colony with a lamellar mixed structure consisting of alternating lamellae of solute-depleted matrix and Ni Mn phase formed at the original grain boundaries and advanced into the grain interiors during discontinuous precipitation. Because of the high strength of discontinuous precipitation colony, the strength of Cu-20Ni-20 Mn alloy increased. The strength of Cu-20Ni-20 Mn alloy was slightly enhanced because of the dispersion strengthening by the Ni Mn phase precipitates depressively on matrix during continuous precipitation. The peak hardness aged at 350℃(discontinuous precipitation was the predominant mechanism) and 450 ℃(continuous precipitation was the predominant mechanism) were 451 Hv and 436 Hv, respectively. It was indicated that the effect of two phase separation mode on strength of Cu-20Ni-20 Mn alloy was nearly the same. An ordered phase with L10 type fct structure, maintaining coherent with the matrix, was observed in aged Cu-20Ni-20 Mn alloy by TEM. And the ordered phase had an orientation relationship with the matrix:(002)matrix∥(001)Ni Mn, [100]matrix∥[100]Ni Mn.The growth rate of discontinuous precipitation colony was related to aging temperature. The activation energy of discontinuous precipitation process was determined to be approximately 72.7 ± 7.2 k J/mol and 94.1±16.8 k J/mol based on the JMAK equation and Aaronson-Liu model, respectively, which was well below the activation energy for volume-bulk diffusion of Ni and Mn in the Cu-rich alloy. This is the reason that discontinuous precipitation colony preferential nucleated and grown up in the grain boundary, and temperature range of the discontinuous precipitation reaction was obviously higher than that continuous precipitation reaction. There was competition between discontinuous precipitation and continuous precipitation. Discontinuous precipitation colony maybe engulf small-scale Ni Mn phase. In addition, continuous precipitations could either partially or completely inhibit the growth of discontinuous precipitation colonies. Obvious inhibition action on the migration of the reaction front of discontinuous precipitation colonies were found by experimental results, owing to the continuous precipitates when their diameters reached up to about 5 nm or larger.The imposition of cold work which leaded to the increase of dislocation density in Cu-20Ni-20 Mn alloy, could not only promote heterogeneous nucleation of the Ni Mn precipitate, but also improve the mechanical properties. The yield strength could reach 807 MPa after 95% cold deformation of Cu-20Ni-20 Mn alloy. Whereas, precipitation hardening would be the predominant strengthening mechanism due to precipitation of Ni Mn phase.The fine grain strengthening was enhanced with decreasing grain size,which lead to the increase of yield strength. The hardness was related to the grain size, corresponding to the Hall-Petch relationship, which could be expressed as Hv=82.6+0.28D-1/2.Based on potential application environments of Cu-20ni-20 Mn alloy, the impact property, wear behavior and low-temperature performance of Cu-20Ni-20 Mn alloy were investigated. The results showed that the strength and Young’s modulus increased slightly at 77 K. The ratios of yield stress and tensile strength values were close to that of Young’s modulus, suggested that the strength of Cu-20Ni-20 Mn alloy was primarily controlled by athermal obstacles. The mechanical properties and lattice constant of Cu-20Ni-20 Mn alloy were constant before or after the deep cryogenic treatment. The impact energy of Cu-20Ni-20 Mn alloy obviously decreased with prolonging the aging time at 450℃. The impact energy changed slightly at low temperatures or after deep cryogenic treatment. The friction coefficient between graphite and Cu-20Ni-20 Mn alloy for different aging treatment was constant(about 0.9).